AbstractThis article is firstly an introduction into query languages
for the Semantic Web, secondly an in-depth comparison of the languages
introduced. Only RDF query languages are considered because, as of the
writing of this paper, query languages for other Semantic Web data
modeling formalisms, especially OWL, are still an open research issue, and
only a very small number of, furthermore incomplete, proposals for
querying Semantic Web data modeled after other formalisms than RDF
exist. The limitation to a few RDF query languages is motivated both by
the objective of an in-depth comparison of the languages addressed and by
space limitations. During the three years before the writing of this
article, more than three dozen proposals for RDF query languages have been
published! Not only such a large number, but also the often immature
nature of the proposals makes the focus on few, but representative
languages a necessary condition for a non-trivial comparison.
For this article, the following RDF query languages have been, admittedly
subjectively, selected: Firstly, the .relational. or .pattern-based. query
languages SPARQL, RQL, TRIPLE, and Xcerpt; secondly the reactive rule
query language Algae; thirdly and last the .navigational access. query
language Versa. Although subjective, this choice is arguably a good
coverage of the diverse language paradigms considered for querying RDF
data. It is the authors. hope and expectation, that this comparison will
motivate and trigger further similar studies, thus completing the present
article and overcoming its limitation.